The Volts battery pack is rated at 16kWhrs. However, to protect the battery pack it isn't deep discharged and is only used to the 8kWhr. limit.
Let assume though that the battery is fully used to get the maximum range the Volt can achieve. The reference states that GM have quoted the petrol engine/generator combo as "The electricity, at around 50 kw, serves primarily to propel the car.". So, they are using 50kW to propel the car under the petrol cycle. As 1hp=746W, 50kW is equivalent to 67hp (a reasonable ball park figure for bhp rating for an equivalent standard internal combustion car engine would obviously need to be higher to allow for losses to through the drive train). Given that the car will take just as much energy to propel it normally, we can assume it needs 50kW during the electric only part of the cycle. So, 8kWhrs of battery would last 8/50 hrs or approximately 9.6 minutes. Assuming the car is averaging 40mph in commuter traffic, this gives an electric only range of 6.4 miles. Alternatively, for the Chevy Volt to return a range of 40 miles during this 9.6 minutes, it would need to be travelling at a fairly brisk 250mph.
This also means at a 6.4 miles electric range from a 50mpg petrol cycle car, the Volt is saving 0.12 gallons of fuel per journey in real world situations using the figures that GM are providing or approximately 1 litre per journey. This is not really planet saving stuff and nowhere near GM's claims or even worthy of all the hype that this vehicle is currently being given or generating, most of it through Viral Advertising I suspect.
Are GM making up the figures as they go along?
Ref. for power used in propelling Chevy Volt: